The present invention relates to an unloading valve used in a hydraulic circuit for, e.g., a construction machine, and more particularly to an unloading valve having a spool which is formed with a flange for restricting slide movement of the spool and properly positioning the spool in its neutral state.
Various hydraulic control valves are conventionally used in a hydraulic circuit for, e.g., a construction machine, and one of those valves is an unloading valve. An unloading valve has the function of releasing the pressure in a hydraulic line when that pressure rises above the reference pressure in excess of a setting value, thereby keeping the pressure in the hydraulic line from rising above the reference pressure in excess of a certain value.
FIG. 5 shows one example of an unloading valve. In FIG. 5, the unloading valve has a valve body 2 in which a spool bore 3 is formed. A first pressure chamber 4 and a second pressure chamber 5 are formed at opposite ends of the spool bore 3, and a reservoir port 6 is formed in an intermediate portion of the spool bore 3. A spool 1 is slidably inserted in the spool bore 3. A hydraulic fluid, of which pressure is to be controlled, is introduced to the first pressure chamber 4, and a hydraulic fluid under a pressure as a reference for control is introduced to the second pressure chamber 5. Further, a flange 7 is formed at an end of the spool 1 on the side of the second pressure chamber 5, and serves as a stopper coming into contact with an abutment surface 8 provided in the valve body 2, thereby preventing the spool 1 from slipping off. The flange 7 also functions as a spring receiver and supports a spring 10 disposed in the second pressure chamber 5.
Further, a transverse hole 1b is bored in the spool 1 near its end on the side of the first pressure chamber 4, and is communicated with the first pressure chamber 4 through a longitudinal hole 1a. 
Assuming in the above-described construction that the pressure of the hydraulic fluid introduced to the first pressure chamber 4 is P1, the pressure of the hydraulic fluid introduced to the second pressure chamber 5 is P2, and the pressing force of the spring 10 is Fk, the spool 1 of the unloading valve operates so as to satisfy the following hydraulic balance formula:
P1xc2x7A=P2xc2x7A+Fkxe2x80x83xe2x80x83(1)
Herein, A represents, as shown in FIG. 6, an effective pressure bearing area of each of pressure bearing portions of the spool 1, which are positioned in the first pressure chamber 4 and the second pressure chamber 5. More specifically, on the side of the second pressure chamber 5, a pressure bearing area dA of an annular portion of the flange 7 on one side thereof is the same as that of a corresponding portion of the flange 7 on the opposite side (i.e., the side facing the abutment surface 8). Therefore, pressing forces imposed on those peripheral portions on both the sides of the flange are canceled and the pressure bearing area dA of the annular portion of the flange does not take part in the operation of the spool 1.
Then, when the pressure P1 rises in the formula (1) to such an extent that the differential pressure between the pressure P1 and the pressure P2 exceeds a hydraulic converted value (setting pressure) of the spring force Fk, the hydraulic balance expressed by the formula (1) is lost, whereupon the spool 1 moves to the left in the drawing and the hydraulic fluid in the first pressure chamber 4 is released to the reservoir port 6 through the longitudinal hole 1a. Thus, the unloading valve is opened and the pressure P1 is lowered. As a result, the pressure P1 is controlled to be held higher than the pressure P2 by the setting pressure of the spring 10.
Further, FIG. 8 of U.S. Pat. No. 5,305,789 discloses an unloading valve provided with a flange having an outer diameter set as small as possible.
However, the above-described prior art has problems as follows.
The spool 1 of the unloading valve is, as described above, provided with the flange 7. For the necessity of precisely positioning the spool in its neutral state, the flange 7 and the abutment surface 8 of the unloading valve body 2, which comes into contact with the flange 7, are both finished into high flatness. Because of such high flatness, the flange 7 and the abutment surface 8 tend to intimately contact with each other. Further, if the hydraulic fluid under a high pressure is present in the second pressure chamber 5, a very small amount of the hydraulic fluid enters the interface between the flange 7 and the abutment surface 8, whereby adhesion between the flange 7 and the abutment surface 8 is promoted. When the flange 7 and the abutment surface 8 are brought into such a tightly close contact condition, the pressure P2 no longer acts upon one of the opposite sides of the flange 7, i.e., the side of the flange 7 facing the abutment surface 8. Hence, the pressure bearing area dA of the annular portion of the flange 7 becomes effective in pressure balance and the pressure bearing area on the side of the second pressure chamber 5 is increased from A to A+dA.
Accordingly, the hydraulic balance formula for the spool 1 is expressed by:
P1xc2x7A=P2xc2x7(A+dA)+Fkxe2x80x83xe2x80x83(2)
Comparing the formula (1) and (2) with each other, it is seen that, as a result of the flange 7 coming into tightly close contact with the abutment surface 8, the cracking pressure, at which the spool 1 starts moving to the left in the drawing and the unloading valve is opened, is increased by a value of P2xc2x7dA/A. Consequently, the proper operation of the unloading valve is impeded.
In the unloading valve shown in FIG. 8 of U.S. Pat. No. 5,305,789, the outer diameter of the flange 7 is set as small as possible. FIG. 7 shows a modification of the unloading valve shown in FIGS. 5 and 6, which has the flange 7 having the outer diameter set as small as possible based on the concept of the US Patent. In FIG. 7, character 7A denotes a flange having a reduced outer diameter. With the flange 7A having the reduced outer diameter, an increase in pressure bearing area of the flange 7A, which occurs on the side facing the interior of the second pressure chamber 5 when the flange 7A comes into tightly close contact with the abutment surface 8, is reduced from dA to dAxe2x80x2 and a rise of the cracking pressure is also suppressed. It is however impossible to perfectly prevent a rise of the cracking pressure, which occurs upon the flange 7A coming into tightly close contact with the abutment surface 8. Reducing the outer diameter of the flange 7A causes another problem in that the strength of the flange 7A deteriorates.
For example, when an unloading valve is used in a hydraulic circuit for load sensing control (hereinafter referred to as xe2x80x9cLS controlxe2x80x9d) of a hydraulic pump, the pressure P1 is given by a delivery pressure of the hydraulic pump and the pressure P2 is given by a load pressure of an actuator (maximum load pressure). Then, the unloading valve functions to hold the differential pressure between the pump delivery pressure and the maximum load pressure at a setting value. In that case, a high load pressure at a level of, e.g., 300 MPa may act momentarily in the second pressure chamber 5 at the startup of the actuator. This means that the flange 7A of the spool 1 hits against the abutment surface 8 under an action of the high pressure of 300 MPa and is subjected to a great impact force. For that reason, there occurs a risk that the strength of the flange 7A having the reduced outer diameter is deteriorated to such an extent as not withstanding the great impact force, and the flange 7A is broken.
It is an object of the present invention to provide an unloading valve capable of preventing a rise of the cracking pressure, which occurs upon a flange coming into tightly close contact with an abutment surface, without reducing the strength of the flange.
(1) To achieve the above object, the present invention provides an unloading valve comprising a valve body having a first pressure chamber, a second pressure chamber and a spool bore; a spool having a spool body inserted in the spool bore such that one end of the spool is positioned in the first pressure chamber and the other end of the spool is positioned in the second pressure chamber; a flange provided at the end of the spool on the side of the second pressure chamber and coming into contact with an abutment surface formed on the side of the valve body, thereby properly positioning the spool body in a neutral state; and a spring disposed in the second pressure chamber and biasing the spool such that the flange contacts the abutment surface on the side of the valve body, the spool being moved toward the side of the second pressure chamber when a pressure in the first pressure chamber rises above a pressure in the second pressure chamber in excess of a setting value of the spring, thereby releasing the pressure in the first pressure chamber to a reservoir, wherein at least one of the valve body and the spool body includes hydraulic pressure guiding means for introducing a hydraulic fluid under the same pressure as that in the second pressure chamber to an interface between the flange and the abutment surface on the side of the valve body.
By providing the hydraulic pressure guiding means which introduces the hydraulic fluid under the same pressure as that in the second pressure chamber to the interface between the flange and the abutment surface on the side of the valve body, even when the pressure in the second pressure chamber rises to a high level, the interface between the flange and the abutment surface is also subjected to the pressure at the same level, whereby the flange and the abutment surface are avoided from coming into tightly close contact with each other. As a result, a rise of the cracking pressure, which occurs upon the flange coming into tightly close contact with the abutment surface, can be prevented without reducing the strength of the flange.
(2) In the unloading valve of above (1), preferably, the hydraulic pressure guiding means comprises a circumferential fluid groove formed in an inner peripheral surface of the spool bore in the valve body at a position near the abutment surface, and a fluid passage for introducing the hydraulic fluid under the same pressure as that in the second pressure chamber to the circumferential fluid groove.
With that feature, the hydraulic fluid introduced to the circumferential fluid groove reaches the interface between the flange and the abutment surface through a small gap between sliding surfaces of the spool bore and the spool body. Therefore, the interface between the flange and the abutment surface is also subjected to the same pressure as that in the second pressure chamber.
(3) In the unloading valve of above (1), preferably, the hydraulic pressure guiding means comprises a circumferential fluid groove formed in an outer peripheral surface of the spool body of the spool at a position near the abutment surface, and a fluid passage for introducing the hydraulic fluid under the same pressure as that in the second pressure chamber to the circumferential fluid groove.
With that feature, the hydraulic fluid introduced to the circumferential fluid groove reaches the interface between the flange and the abutment surface through a small gap between sliding surfaces of the spool bore and the spool body. Therefore, the interface between the flange and the abutment surface is also subjected to the same pressure as that in the second pressure chamber.
(4) In the unloading valve of above (1), preferably, the hydraulic pressure guiding means comprises a circumferential fluid groove formed at an opening end of the spool bore in the valve body on the side of the second pressure chamber, and a fluid passage for introducing the hydraulic fluid under the same pressure as that in the second pressure chamber to the circumferential fluid groove.
With that feature, the hydraulic fluid introduced to the circumferential fluid groove is directly introduced to the interface between the flange and the abutment surface, whereby that interface is also subjected to the same pressure as that in the second pressure chamber.
(5) In the unloading valve of above (1), preferably, the hydraulic pressure guiding means comprises fluid grooves formed in the abutment surface on the side of the valve body and always partly opened to the second pressure chamber.
With that feature, the hydraulic fluid under the same pressure as that in the second pressure chamber is directly introduced to the interface between the flange and the abutment surface.